Process of coating ferrous silicon magnetic material



Feb. 5, 1946. H. M. ELSEY ET AL PROCESS OF COATING FERROUS SILICON MAGNETIC MATERIAL Filed July 24, 1941 WITNESSES: H dIWIEY/ENTORS d c orvar fey an v fora C. H fmafl. 6 B

Patented Feb. 5, 1946 PROCESS OF COATING FERROUS SILICON MAGNETIC MATERIAL Howard M. Elsey, Oakmont, and Clifford C.

Horstman, Sharpsville, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 24, 1941 Serial No. 403,878

3 Claims.

This invention relates to transformers, and more particularly, to a magnetic core comprising a plurality of laminations.

The object of this invention is to provide an insulating film on heets of magnetic material.

A further object of this invention is to provide a coating for sheets of magnetic material, which coating functions both as a refractory to separate the sheets during annealing and reacts at the surface of the magnetic material during annealing to form a tenaciously adhering insulating film.

Other objects of the invention will, in part, be obvious, and will, in part, appear hereinafter.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing in which:

Figure 1 is a diagram showing schematically apparatus for practicing a process for treating sheets of magnetic iron in accordance with this invention;

Fig. 1A is a schematic diagram of apparatus for practicing a process for treating a magnetic core and preparing it for a'transformer structure; and

Fig. 2 is a sectional view of a plurality of sheets of magnetic material separated by a refractory coating.

In the building of transformers having high electrical efficiency, the laminations of the core need to be electrically insulated from one another. According to this invention the core laminations carry an integral insulating film which will meet predetermined requirements It has been discovered that, in order to produce a laminated magnetic core wound from continuous strip magnetic material and achieve inthe finished core iron losses no higher than measured by an Epstein test on the material used, it is necessary to provide on the surface of the magnetic sheet an insulating film which functions as interlamination insulation to give low eddy-current losses. In order to keep these losses at predetermined values, the insulating film in particular must have a high degree of electrical resistance, it must adhere tenaciously to the laminations of the magnetic core, and must withstand thermal and mechanical stresses developed in the laminations when the apparatus in which the laminated material is embodied is in operation.

In building a transformer having a wound magnetic core which meets the standard of high efliciency herein set forth, namely, iron losses no higher than obtained by-an Epstein test of material used, it has been found that the type of insulating film separating the laminations, in order to obtain low eddy-current losses, is a critical factor; In particular, the reduction of core losses to 0.8 watt per pound, or less, at 15,000 gauss at cycles per second requires interlamination insulation having good dielectric characteristics. Furthermore, in order to meet design and manufacturing considerations, the laminated material for magnetic cores should be capable of being assembled into a core with a space factor of over preferably 94% or 95%.

While it would be possible to insulate core laminations from each other by introducing a large amount of known insulating materials between laminations to give low eddy-current losses, the space factor consideration imposes a severe requirement which is difficult to meet by any known insulating material that will also meet other requirements of this type of structure, namely, tenacious adherence to laminations and capacity to withstanding thermal and mechanical stresses developed during processing.

By means of this invention, an insulating film having characteristics which will meet the above requirements is produced. Briefly, by following the process herein disclosed, an insulating film composed of a complex glass-like reaction product of iron oxide, magnesia and silica is formed directly and integrally on the surface of the laminations of magnetic material during a heat treatment to develop the optimum magnetic and low loss characteristics in the core material. The

- exact chemical composition of this glass-like complex of iron oxide, magnesia and silica is not fully known at the present time. By microscopic and chemical analysis, it is known that the film includes particles of a refractory, such as magnesia, bonded tothe siliceous glass. By following the method of producing it, as given herein in detail, this film with the predetermined characteristics may be secured consistently regardless of a lack of a complete knowledge of its chemical or physical characteristics.

There are certain mechanical stresses which will be imposed upon the film in the core building perations which will be disclosed hereinafter.

Specifically, these considerations require a tenacious adherence of the film to the strip of magnetic material which permits it to be bent about dust-free and will not scale or break up during manipulation. In addition, the film must have such properties that the magnetic material may be exposed to temperatures of at least 900 C. or higher without the film fusing or deteriorating.

The insulating film of this invention adequately meets the above requirements as called for by the particular core construction that will be detailed in the present application. In addition, the film is suitable for application to magnetic material which is to be assembled into other types of cores having less exacting mechanical and electrical requirements imposed thereon.

The particular transformer core construction herein disclosed is especially desirable for applications embodying magnetic material with superior qualities of high permeability and low loss produced by aligning most of the grains in a single grain or crystal direction. This material is a silicon iron which has approximately 3 4% silicon, and is so treated that the grains of the silicon iron are given a preferred magnetic orientation. Such a material having preferred grain orientation may be conveniently produced by a series of drastic cold-rolling reductions of the order of 40% to 85% with annealing between each series of rolling, as disclosed in the patent to Cole et al. No. 2,158,065. Other methods of securing the preferred grain orientation are known in the art. The preferred orientated magnetic material is generally produced with a low carbon content of the order of 0.005%, in order to minimize any ageing or deterioration in the magnetic efiiciency of the material. Where the loss and permeability requirements are not as critical as those referred to above, other high silicon irons in strip form may be subjected to the treatment producing this silicate insulating film and used in any desired type of electrical apparatus.

The magnetic material which consists of the high silicon iron with a preferred grain orientation is generally obtained in large rolls or in sheets. The roll or sheet material must be subjected to a heat treatment at a temperature of from 900 C. to 1300 C., in order to obtain optimum magnetic properties. It is further required in a satisfactory and practical heat treatment to produce this result that the material be subjected to temperatures of this order in rolls or in stacks of sheets Without the sheets in the stack or turns of the roll welding to each other. At the high temperatures involved in this heat treatment, the sheet iron should necessarily carry a refractory of some sort to prevent such welding.

It has been discovered that a selected refractory composition applied to the sheets of silicon iron in the manner herein disclosed will accomplish the following dual function. First, the refractory will operate to cause a separation of the sheets during the heat treatment operation without welding or bonding. Second, under the predetermined conditions herein disclosed, the refractory will cooperate with the surface of the sheet iron to produce an insulating film having the highly desirable mechanical and electrical characteristics herein disclosed.

In the specification and claims, the term stacking is intended to denote both the formation of rolls consisting of a plurality of laminations of material from long strips and the piling of individual sheets into a large bundle of laminations.

Referring to Fig. 1 of the drawing, a schematic showing of the sequence oi operations required chemical properties.

to produce heat treated silicon iron sheets with an insulating film thereon is shown. Long strips of sheet material may be subjected to the same operations.

Sheets of silicon iron such as are produced by the series of cold rolling operations disclosed in the Cole et al. patent, have substantially unoxidized surfaces. The sheet of material is kept clean and free of oxides, usually by films of oil on the surfaces. These oil films may be removed by washing in an alkali bath or a bath containing an oil solvent.

Thereafter the cleaned sheets are subjected to a coating operation in the apparatus I0 of Fig. 1. Two coating rolls [2 operate simultaneously upon both surfaces of a sheet or strip of material. These rolls convey a thin coating of the composition I6 and 20 from the trough l4 and the drip pan I8, respectively. No particular effort is made at this stage to produce any uniform or predetermined thickness of coating.

The coating applying device may assume other forms than as shown at H]. Strips of the silicon iron may be dipped into a tank containing the composition and withdrawn with a thick layer of composition on both surfaces. The composition may be sprayed upon the sheets in another modification.

The refractory composition I B and 20 which has been found to produce an insulating film at the completion of the process is preferably a hydrated magnesium oxide suspension. There are available on the market several types of magnesium oxide graded on their mechanical and An extremely fine powder with good purity is that which is known as U. S. P. extra light. This particular material has the characteristics of being rapidly hydrated in wa-- ter. Other available magnesium oxides suitable for use in this invention are not as finely pulverized and their chemical composition indicates the presence of a considerable amount of oxides other than magnesium oxide. The following table indicates the approximate chemical analysis of three magnesium oxides available on the market:

Constituent extra light A B 90. 2 89. 76 84. 46 0.28 2 78 1.25 0. 02 .01 .01 7. 3 3. 67 & 22 2. 1 l. 13 5. 08 0.10 2. 65 0. 98

For the purpose of this invention, the extra light magnesium oxide has certain desirable characteristics in that it forms a thick hydrate or suspension readily when agitated in the desired amount of water. This suspension has very good adhesive characteristics when applied to the strips of sheet material. It has been found, however, that it is so low in silica, that, in order to produce a satisfactory complex magnesium silicate glass film on the sheets of magnetic material during the heat treatment operations, silica should be added. Accordingly, from 2% to 10% of silica fiour is added to the weight of magnesium oxide in order to bring up its silica content. It will be noticed that magnesium oxides A and B have a greater amount of silica than that of the U. S. P. extra light. The amount of silica in magnesia A and B is satisfactory insofar as film producing properties are concerned and good films have been produced from both A and B without silica additions. In some instances, however, the addition of from 2% to 5% of silica flour to composition B has produced beneficial results. Slight silica additions to composition A may be beneficial. I

When the term silica" is used in the claims in describing the coating composition, it is intended to include silicon dioxide either as added to the magnesia or as naturally present in the magnesia as an impurity.

In addition to the magnesia and silica flour, it has been found that the composition will not form an insulating film on silicon iron sheets in the absence of oxidized surface metal. In the patent application of C. C. Horstman and W. H. Brandt, Serial No. 407,200, filed August 16, 1941, and copending with the present application, the sheets of silicon iron were pre-oxidized by a heattreatment in air.

According to this invention, pre-oxidization of sheet silicon iron is not necessary to successfully produce a silicate film on the sheets. It has been discovered that an oxidizing agent may be added to the coating composition to accomplish the same result. During the annealing heat-treatment, the oxidizing agent will cooperate with the silicon iron sheet to produce a predetermined quantity of metallic oxides.

Several materials have been found to give saiisfactory results when added to the refractory coating composition to act as oxidizing agents. Finely pulverized calcium carbonate, hydrated lime, or limestone may be added to the magnesia base coating composition. During the annealing heat-treatment, the calicium carbonate in the coating will dissociate to give off carbon-dioxide gas. At the high temperatures of annealing the carbon-dioxide is oxidizing to the silicon iron. By adding calculated amounts of calcium carbonate, any predetermined degree of oxidation of the silicon iron may be secured.

Other carbonates of the alkaline earth metals may be added to the coating composition to achieve this result.

In using carbonate additions to cause predetermined oxidation of the sheets of silicon iron, the subsequent heat treatment should be carefully conducted in order that the carbon dioxide gas does not react to carburize the iron and increase the carbon content above the 0.005% standard normally desired.

Hydrated lime typifies the preferred class of the hydroxides of the alkaline metals for use as oxidizing additions to the coating composition. Calcium hydroxide, barium hydroxide, and strontium hydroxide have been particularly successful oxidizing agents for the sheets. Under the high temperature conditions of the annealing operation, calcium hydroxide, for example, will decompose to produce water vapor. The water vapor will react with the surfaces of the sheets of silicon iron to cause oxidation thereof.

For use in the particular annealing cycle herein disclosed to produce thin films, additions of from 2% to of the oxidizing a ent have been satisfactory. Larger amounts of oxidizing agent may be added to the magnesia in some cases.

The effect of the oxidizing agent is to produce oxidized surface metal such as silicon dioxide and iron oxide for causing a reaction to occur with the magnesia and silica. There is some evidence that in the subsequent annealing process using hydrogen gas, the iron oxide is reduced to iron in the form of minute particles. For this reason, oxidation 01' the sheets beyond that required for a good insulating film is to be avoided. The presence of excessive amounts of reduced iron may be detrimental to the insulating effect of the A convenient method of producing a suspension of magnesia with or without silica and containing the oxidizing agent is as follows. Approximately 500 pounds of the finely pulverized magnesia powder is placed within a larger agitator containing'500 gallons of water. The a itator is put into operation for one-half hour in order to cause a thorough mixing of the liquid with the magnesium oxide powder. The thoroughly agitated mixture is then allowed to stand for a period of from three to fifteen hours, or

. longer, in order to thoroughly hydrate the magnesium oxide. U. S. P. extra light magnesia will hydrate in three hours, the other compositions will require the longer time. At the end of this period approximately 500 gallons more of water are added in order to make 1000 gallons of suspension. This additional amount of water is blended with the hydrate by agitating for an additional one-half hour.

The resulting material is a rather thick cream which contains from 7 /2% to 8% hydroxide. It

has been found that from 4% to 8% of hydroxide suspension produced the best coatings. Less than 4% of hydrated magnesia results in a thin watery material which readily runs off the sheets being coated and is without adequate adhesion. If over 8% of hydrated magnesia is present, the material is too thick to be applied in satisfactory coatings. In some cases, a slight amount of ball milling of the magnesium oxide, the silica fiour, and the oxidizing agent, such as calcium hydroxide in water, will cause a better suspension of the whole in the hydrated magnesia solution. The ball milling composition is more stable than that produced by simply agitating the solids in water.

In using the U. S. P. extra light magnesia with approximately 5% silica fiour addition thereto and about 5% calcium hydroxide, it has been found that approximately 10 pounds of the 6% hydrated magnesium oxide per ton of material will give a coating productive of the best results. In this case, the silicon sheet iron is approximately 13 mils thick. When less than 5% silica is present in the U. S. P. extra light magnesia, for example, approximately 2%, then up to 20 pounds per ton of the magnesia should be applied in the coating to produce a good insulating film.

In using the magnesia of composition A or B, due to their'relatively coarser structure and less satisfactory hydration, which appears to be a suspension rather than a hydrate, it has been found that approximately 30 pounds per ton of compositions A and B may be required per ton of 13 mil sheet being coated to produce a good film;

The coating composition applie by rolls i2 is graded in thickness and amount by subjecting the strips to the action of soft rubber squeeze rolls 22. The soft rubber squeeze rolls 22 may be operated toproduce closely calculated amounts of composition per ton of sheet material. The excess material squeezed off the sheets by rolls 22 may be returned to trough l4.

The strips of silicon iron, after coating at H and squeeze rolling at 22, are dried within an oven 24 at a temperature of below 300 C. It

is desirable to stay below 300 C. in drying the coating on the sheets of pre-oxidized silicon iron, inasmuch as iron may oxidize excessively at temperatures above 300 C. in the presence of large quantities of moisture. The purpose of the drying treatment is to cause both evaporation of the free water in the coating and in addition, it is believed to cause at least a part of the magnesium hydroxide on the sheets to dehydrate to the oxide. Temperatures of from 200 C. to about 300 C. have been found satisfactory for this result. The drying should be slowly conducted in order to prevent heavier coating from flaking. Slow drying will result in these heavy coatings tightly adhering to the sheets without loss of material. The coated strip, after the drying operation, has a thin layer of magnesium oxide with some magnesium hydroxide present which adheres satisfactorily for the further operations on the material.

The strip of coated and dried silicon iron is placed within the annealing furnace 2 6. Long strips of material issuing from the drying furname 25 are preferably coiled into rolls for ease in placing within the annealing furnace 26. Small individual sheets of silicon iron produced by the previous steps of the process are preferably stacked in large bundles prior to placing within the furnace 26. Due to the length of time consumed in the annealing process and the critical atmosphere requirements, it is customary to anneal stacks of material weighing several tons.

The temperatures within the annealing furnace may range from 900 C. to 1300 C. in order to produce proper grain growth and cause all the materials within the sheets to assume the preferred grain orientation necessary to produce predetermined magnezycquality. For best results, it has been found that the furnace 26 should have a. reducing atmosphere, preferably hydrogen gas with a dew point of approximately C. or less. The period for heat treatment is from 1 to hours, depending upon the temperature and the size of the stack or coils of the material. At 1300 C. satisfactory orientation of all the material in the sheets is obtained within a few hours. At 900 C. the longer heat treatment time is required.

When subjected to the annealing temperatures within furnace 2B, the coating between the sheets of silicon iron behaves as follows: The oxidizing agent begins to decompose at some temperature below 900 C. Gas or vapor, such as carbon dioxide or water, resulting from the decomposition, immediately causes surface oxidation of the sheet material. The alkaline earth metal oxide which remains after the oxidizing gas is emitted does no harm to the subsequent chemical reaction and is believed to even assist in the production of the insulating film.

At the surface of the sheets, particularly at temperatures of 1150 C. and higher, the iron oxide begins to react in the presence of the magnesium oxide and silica in the refractory coating immediately adjacent thereto. Some of the silicon in the silicon-iron sheet will be involved in this reaction in the form of silica. A complex silicate glass comprising iron oxide, magnesium oxide and silicon oxide is formed immediately adjacent to the surfaces of the sheets of material and on cooling, adheres tenaciously thereto.

In some cases, a substantial proportion of the silicate component of the film is derived from the metallic silicon in the sheets which is oxidized during the reaction and diffuses to the surface of the sheets of silica. Tests on the silicon content of the sheets substantiate this fact.

The reaction between the particles or magnesia, silica and iron oxide film on the silicon iron proceeds relatively slowly due to the viscosity of the fluid reaction product. It is possible that the reaction may not proceed to completion within the annealing times stated. However, the reaction does result in a sufilcient glass being produced to provide for a good film on the sheets of iron.

Referring to Fig. 2 of the drawing, there is illustrated a cross-section of a portion of a stack of laminations after annealing heat treatment. The refractory coating 04 remote from the surfaces of the sheets and not subject to the influence of the iron oxide remains substantially unchanged. The unreacted portions 64 of the coating function to separate the sheets 60 and prevent welding or bonding. Easy separation of the sheets after annealing is, therefore, promoted by the coated layer 84. Immediately adjacent the interface between the sheets 60 and the unchanged surfaces of the coating 84 is the thin layer 02 of complex silicate glass. This glass film has a thickness of about 0.0001" or less.

Examination of the glassy film on these sheets after removal from the annealing furnace and lightly brushing away all of the loose coating 64 shows that the film 62 consists of a complex glass of iron oxide-m nesium silicate. Chemical analysis of the film 62 is quite difficult due to its thinness and adherence to the silicon iron sheet. The analysis of a film sample secured from a sheet after brushing of! all loose particles showed that the silica and magnesia were combined in a one to one ratio. Approximately 4.4% of iron was present, probably as iron oxide. Other film samples had from 1.4% to 5.7% iron.

In addition, microscopic examination of the glass film exhibits particles of the magnesia bonded thereto. The particles are quite small and are arranged in various positions in the film. Some of the particles of magnesia are practically surrounded by the glass, while others are merely attached to the glass at one point. It is not known whether some surface reaction has occurred on the particles. It is believed to be desirable to permit as many as possible of the bonded magnesia particles to remain part of the coatingfi. Severe brushing of the coating may dislodge and tear off these particles of magnesia as well as shatter the glass film, and is accordingly not recommended.

Experimental tests have shown that in the absence of an oxidized metal surface, such as is secured by oxidizing additions to the coating composition, substantially no glass film forming reaction occurs in coating 64. It has been found necessary to produce such a thin metal oxide layer on the silicon iron to cause the adjacent portions of coating 64 to fuse into a glass. By localizing the metal oxide to the surfaces of the sheets, glass-like film formation occurs only at this place.

The temperature has been found to exert a critical eiTect on the formation and physical characteristics of the complex glass film. Below 1150" C. the film does not form rapidly and it tends to be quite thin and fragile. At 1150 C., or slightly above, the reaction between the iron oxide, magnesia and silica proceeds with greater speed. The glass-like film is thicker. It contains quantities of unreacted magnesia particles bonded thereto. In most instances, the film pro- The adhesion of the magnesia particles enhances the electrical insulating characteristics. It is, therefore, advisable not to brush this type of film severely and dislodge the bonded magnesia particles.

At 1200 C. the chemical reaction of iron oxide magnesia and silica is quite rapid and more complete than at lower temperatures. The film is heavier and more durable than those produced at 1150 C. or lower. This film has numerous particles of magnesia embedded and bonded thereto. It may be brushed thoroughly to remove the loose layer 04 without impairing its insulating characteristics. The sheet with this film may be bent about radii of one-fourth inch without separation thereof. The sheet may be rolled vigorously in subsequent shaping operations without causing noticeable loss of the film or insulating value. It is, accordingly, preferred to anneal the coated sheets at 1150 C. to 1200 C. or higher, in order to produce the superior film.

An example of the electric characteristics of the film produced at several temperatures is the following table:

Median resistivity r lamination Temperature of anneal S. P. extra light with 5% CaO and 5% SiO;

Ohms/cm. 1225 C 2.0 1200 C 0.4 1100 C 0.1

It will be seen that the coating composition accomplishes the dual function which has been found necessary in this invention. The sheets of silicon iron are prevented from welding or fusing together during the annealing while the coating chemically reacts to produce a predetermined thickness of insulating film which is tenaciously adherent to the laminations of iron.

After a heat treatment within furnace 26 sufficient to produce preferred grain orientation and optimum magnetic properties in the silicon iron, the material is cooled to 500 C. in the furnace before sheets are exposed to the atmosphere. After completely cooling, the sheets are subjected to a brushing operation by means of wet or dry brushes 28. This operation brushes away all of the loose coating 64 and any scale or non-adhering material present on the sheets. The individual sheets have a gray appearance due to the presence of the film of silicate 82.

In order to determine whether a satisfactory film ha been produced, the sheet may be tested directly for the electrical resistance of the film. A satisfactory criterion of the quality of the film is 'a resistance of one ohm or more per square centimeter when the measuring contacts are given a slight twist to test the abrasion resistance of the film. A film that has resistance of this order or better is satisfactory for building into the highly efiicient transformer core above specified.

Since the strips of silicon iron are generally produced in widths of 24 inches and most transformer cores are of much narrower width than this, the strip must be slit to required width. This is effected by the slitting rolls 30 of suitable design.

The slit silicon iron is then formed into a wound duced at 1150' C. has good electrical resistance.

on a mandrel 32 of desired configuration. To secure the predetermined space factor of over 90%, the strip is wound into a core 34 of suitable thickness under the application of pressure from roll 36. The insulating film obtained by this invention may be subjected to this windin operation without cracking or falling in any way. Very small radii of the order of one-eighth inch at the corners of the mandrel may beemployed.

The wound core 34 is strain annealed in a furnace 38 at a temperature of from 700 C. to 1000 C. in order to remove all the strains induced by the winding operation. The insulating film obtained by the method of this invention will not be impaired by this annealing operation.

In order to embody the wound core 34 into a transformer construction, it has been found desirable to split the wound core into two parts. For the purpose of splitting the core without loosening the laminations from the wound formation, the sequence of operations shown in Fig. 1A is 1 followed.

The wound core is impregnated with a resinous bonding material capable of filling all the spaces between the consolidating laminations into a solid core. This impregnation treatment is conducted within an impregnating tank 40 of any suitable design. For best results, vacuum impregnation may be used.

The resinous material may be either of a thermosetting or of a thermoplastic type. If a thermoplastic resin is used, it is preferred to modify the resin with a proportion of some thermosetting material in order to obtain a more rigid body. Suitable resins for the impregnation are a modified polyvinyl acetate as disclosed in the patent application of James G. Ford, entitled Bonded laminated magnetic material, Serial No. 331,785, filed April 26, 1940.

The impregnated core is placed within the resin heat-treating apparatu 42. The heat treatment within this apparatus is designed to cause evaporation of the resinous solvents and to effect suitable polymerization of the resin to increase its visccsity and hardness. In case of thermosetting resins, the resin is converted into its infusible state. After this heat treatment, the core is a substantially solid body which may be safetly cut into sections without de-lamination.

A satisfactory method of splitting the transformer core consists of a saw 44 and a traveling carriage 46 holding the core 34 to be cut. The core may be out along any predetermined section in order to suit manufacturing requirement. The sections may now be readily assembled into a transformer construction much more easily than would be possible with an uncut wound core.

It has been discovered that the sawing operation produces a number of slivers and burrs. These slivers and burrs will short-circuit the laminations and cause high eddy current loss. Furthermore, when the cut joint is rough, it will introduce a high air gap loss when the core is reassembled. For this reason, the core is preferably subjected to a further finishing operation upon the cut joints. It is desirable to first grind the joints by means of a grinding wheel 48. The core section may be held within a jig 50 within which the core is clamped between the stationary abutment 52 and the movable abutment 54 adjustable by means of screw 56. The grinding wheel 48 will produce a joint whose surfaces are substantially plane and flat.

In addition to the grinding operation, for best results, a subsequent acid etching operation is required to eliminate the finest burrs or slivers which have remained after operating with the grinding wheel. This acid etching operation may be performed without causing injury to the laminations since the resin and glassy film withstand the action of the acid, and only the exposed metal and burrs at the Joint are affected by the acid. Thus, an exceedingly desirable type of joint for transformers is produced.

The ground and etched sections of the core may be thereafter assembled with suitable coils and other electrical elements within the transformer construction 58.

From the preceding description, it will be seen that the complex magnesium silicate iron oxide film will have good electrical resistance, high tenacity, and resistance to the effects of both temperature and mechanical deformation in order to produce the core construction detailed. The strength of the bond between the film and the laminations is as good as that of the resin. In practice, it has been found that the film produced by the operations of Fig. 1 meets these requirements successfully.

The coating applied to the raw material is so controlled that it does not exist as an infusible phase along with a fused phase at the iron sheet surface in the annealing operation. By means of the predetermined addition of oxidizing agent, the thickness and extent of this coating are readily controllable. Thus, good space factor in the transformer core is secured.

Since certain changes in carrying out the above process and certain modifications in the article which embody the invention may be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense. Therefore, it is desired that the invention be interpreted as broadly as possible and that it be limited only by what is expressly the following claims.

We claim as our invention:

1. The method of treating sheets of ferrous silicon magnetic material to provide for annealing the magnetic material and to produce an electrically insulating film on the surfaces of the sheet which comprises, in combination, applying to the surfaces of the substantially unoxidized sheets a set forth in coating composition which is refractory at annealing temperatures, the composition comprising essentially magnesium oxide, from 2% to 10% of a substance capable of oxidizing the ferrous magnetic material selected from the group consisting of metallic carbonates and hydroxides, and a minor proportion of silica, stacking the coated sheets and heat treating the stacked sheets in a reducing atmosphere comprising hydrogen gas at temperatures of 1150 to 1300 C, to eflect annealing of the ferrous magnetic material, the oxidizing substance in the coating composition chemically reacting to oxidize the surface of the sheets to form metallic oxide, the composition adjacent the metallic oxide thereafter chemically reacting to form a silicate film having good electrically insulating properties to provide for low eddy current loss and being tenaciously adherent to maintain the insulation protection on the sheets during subsequent manipulation.

2. The method of treating strips of ferrous magnetic material to provide an electrically insulating film on the surface of the sheets, comprising, in combination, applying to the surfaces of the strips a refractory composition comprising essentially magnesium oxide, a minor proportion of silica, from 2% to 10% of calcium hydroxide stacking the coated strips and heat-treating the stacked strips at temperatures of from 1150 to 1300 C. to effect oxidation of the ferrous magnetic material by the oxidizing substance to form metallic oxide, the coating composition thereafter chemically reacting with the metallic oxide at the interface to produce a siliceous film having good electrically insulating properties and having tenacious adherence to the strips.

3. A liquid coating composition for application to surfaces of sheets of ferrous material to produce a thin insulating film on the surfaces during heat treatment and concurrently providing a refractory sheet separating agent during such heat treatment, the composition comprising parts of magnesium oxide, from 1 to 5 parts of silica and from 2 to 10 parts of calcium hydroxide for oxidizing iron, the composition in water to produce 4% to 8 aqueous magnesium hydroxide suspension.

HOWARD M. ELSEY. CLIFFORD C. HORS'IMAN. 

